1. Field of the Invention
This is a method pertaining to fertilizer chemistry, for leaching or chemical beneficiation of dolomitic phosphate mineral ores. The ore so leached is more desirable for use in conventional manufacture of fertilizer-grade phosphoric acid by acidulation with sulfuric acid. Magnesium hydroxide byproduct is precipitated from the leachate, and pure carbon dioxide can be separated as a second byproduct.
2. Discussion of Prior Art
Although the highest quality US phosphate resources are being depleted, great reserves of dolomitic phosphate ore remain. These deposits are less desirable because of their magnesium content. Magnesium present in ore is produced in the phosphoric acid making it highly viscous so that filtration is difficult. When the phosphoric acid is later reacted with ammonia to form ammonium phosphate fertilizers, magnesium in the acid forms troublesome magnesium ammonium phosphate. This product crystallizes slowly and tends to deposit inside pipes and vessels. Its reversible hydration interferes with handling and weighing of the final fertilizer. A process is needed for the economical removal of magnesium.
Four different magnesium-separation methods have previously been developed to various stages: 1) Flotation using air bubbles brings one mineral to the surface while another sinks. Surfactants help bubbles attach to particle surfaces. 2) Very finely pulverized dense solid material such as ferrosilicon is mixed with water to make a semi-fluid mud of controlled density intermediate between dolomite and phosphatic minerals. The dolomitic fraction tends to rise to the surface with agitation in this "heat media" separation while phosphate sinks. 3) Ion exchange recovers magnesium sulfate from phosphoric acid after it is produced by strong acidulation of ore. 4) Acid leaching dissolves the carbonate minerals calcite and dolomite but not phosphate,
Most prior efforts to remove the dolomite have been directed at flotation (U.S. Pat. Nos. 4,636,303, 4,648,966, 4,804,462, Moudgil et al 1987, Moudgil 1988, Moudgil and Vasudevan 1988, Hanna and Anazia 1990). Unfortunately, the flotation behavior of carbonates closely resembles that of phosphates. Although many combinations of conditioning, surfactants, temperature, ionic strength etc. have been tried, no commercially attractive method has been found. Flotation separation of magnesium has not progressed beyond bench scale in spite of ongoing efforts at University of Florida and elsewhere. In addition, flotation depends on both the chemistry and microscopic surface characteristics of the minerals. Both chemistry and surface characteristics vary from one phosphate deposit to another and between strata in the same deposit, so that many combinations are possible. If a useful flotation method is discovered it may be limited in application to ore types with particular surface characteristics.
Separation by heavy media has progressed to experimental commercial demonstration at a plant owned by International Minerals and Chemicals in Florida. This method is useful only with relatively large particles which move readily through the ferrosilicon mud. Fine particles which inevitably result from ore mining and crushing are discarded, with loss in phosphate values. The large particles which are used are typically composed of both phosphate and carbonate minerals aggregated together into clumps of intermediate density. This interferes with quality of separation.
Flotation and heavy media separate the magnesium by physical means, reducing overall requirement for sulfuric acid. Ion exchange and leaching separate the magnesium by chemical means.
Ion exchange is applied to phosphoric acid after it has been produced by digesting the phosphate ore with strong sulfuric acid. Magnesium is absorbed into an ion exchange resin. The resin is then separated from the phosphoric acid and is treated with additional sulfuric acid to recover magnesium sulfate. This has been known for many years. It is practiced commercially on a small scale. Disadvantages are: 1) relatively low byproduct value for the magnesium sulfate, 2) additional acid requirement for exchange resin regeneration and 3) the difficult filtration of magnesium-containing phosphoric acid is not avoided.
Acid leaching removes the magnesium ahead of the phosphoric acid digester, so that magnesium does not interfere with filtration. Leaching depends on chemical properties of mineral phases in the ore only, and not on microscopic surface structure. It is therefore less sensitive to variation in feed characteristics than is the case with flotation. All of the mined ore can be leached regardless of particle size, so phosphate loss is small. By the methods of this invention, separation is more complete than by any known alternative.
Carbonic acid is a weaker acid than phosphoric acid. Acidity of strength intermediate between carbonic and phosphoric acid can dissolve dolomite and bring its magnesium into solution without dissolving the phosphorus-containing apatite or francolite minerals. Thus purified, phosphate ore can be used in the conventional wet phosacid process.
Weak acids such as acetic acid ionize partially in water. At high concentration, reversible ionization has the effect of limiting and roughly regulating pH. By a suitable choice of weak acid, the leaching solution pH might be made low enough to leach dolomite, but too high to dissolve apatite. The weak acid must be present in molar excess, and pH changes somewhat as it is depleted by reaction.
Several concepts have been explored for dolomite leaching using weak or intermediate-strength acids such as sulfur dioxide, hydroxysulfonic acid, acetic acid, maleic acid or other carboxylic acid (Orlov and Treushchenko 1975, U.S. Pat. No. 4,238,459, SU 469,664). Weak acid leaching has not produced excellent separation due to its inherently imprecise pH control coupled with failure to actively remove carbon dioxide from reaction products.
For example (Hansen, et al 1985) tested leaching with aqueous sulfur dioxide in a gas-tight system. Twelve Variations were tried. Best results removed 65% of the ore magnesium with 3.9% loss of phosphate. Similarly, Abu Elshah et al 1991 tested acetic acid leaching. Their typical result was 60% carbonate removal with 25-30% phosphate loss. When magnesium hydroxide is precipitated from leachates such as these, the co-dissolved phosphate forms magnesium phosphate. Large percentages of magnesium phosphate make the magnesia unsuitable for use in refractories or in production of magnesium metal, the two principal markets for this element.
There is further difficulty when ammonia or ammonium ion is present in the precipitating mixture, as in Soviet patent 366,177. Magnesium ammonium phosphate so formed typically crystallizes slowly and tends to clog filters and deposit in pipes and vessels. This problem is one of the principal motivations for removing magnesium from the phosphate. Such fertilizer also tends to change weight with variation in humidity, and form cakes that interfere with handling.
Weak acids are relatively expensive compared to sulfuric acid. Weak acids can be recovered for reuse as in Soviet patent 469,664, but this adds to process cost and complexity. Sulfuric or some other cheap acid must then be consumed to regenerate the weak acid.